In the state of the art, automatic gearboxes, particularly for motor vehicles, comprise planetary gearsets that are switched by way of friction or shifting elements, such as clutches and brakes and are usually connected with a starting element that is subject to slip-effect and is optionally provided with a lock-up clutch, such as a hydrodynamic torque converter or a fluid clutch.
The Applicant's DE 101 15 983 A1, for example, discloses a multi-speed gearbox with an input shaft that is connected to a front-mounted gearset, with an output shaft that is connected to a rear-mounted gearset with a maximum of seven shifting elements, which can be selectively switched to produce at least seven forward gears without range shifting. The front-mounted gearset consists of a shiftable or non-shiftable planetary gearset, or a maximum of two non-shiftable, coupled planetary gearsets. The rear-mounted gearset is configured as a two-carrier, four-shaft gearbox with two shiftable planetary gearsets, and has four free shafts. The first free shaft of this two-carrier, four-shaft gearbox is connected to the first shifting element, the second free shaft with the second and third shifting elements, the third free shaft with the fourth and fifth shifting elements, and the fourth free shaft is connected to the output shaft. For a multi-speed gearbox with a total of six shifting elements, the invention proposes to additionally connect the third free shaft or the first free shaft of the rear-mounted gearbox with a sixth shifting element. For a multi-speed gearbox with a total of seven shifting elements, the invention proposes to additionally connect the third free shaft with a sixth shifting element, and to additionally connect the first free shaft with a seventh shifting element.
Several other multi-speed gearboxes are known, for example from the applicant's DE 101 15 995 A1, which discloses four shiftable, coupled planetary gearsets and six or seven friction-locking shifting elements, whose selective engagement transfers a revolution speed of an input shaft of the gearbox to an output shaft of the gearbox in such a way that nine or eleven forward gears and at least one reverse gear can be implemented. Depending on the gear arrangement, in each gear two or three shifting elements are engaged, and when switching from one gear into the next higher gear or the next lower gear, only one engaged shifting element is disengaged, and one shifting element, which was previously disengaged, is engaged, in order to avoid range shifting.
In addition, the applicant's non-published generic patent application DE 10 2005 002 337.1 discloses a multi-speed gearbox with an input shaft, an output shaft, four coupled single planetary gearsets and five shifting elements, by way of which eight forward gears can be shifted without range shifting in such a way that when switching from one forward gear into the respective next following higher or lower forward gear, only one of the previously engaged shifting elements is disengaged, and only one of the previously disengaged switching elements is engaged. The multi-speed gearbox also has a reverse gear. In each of the forward gears, and in the reverse gear, three shifting elements are engaged. With respect to the kinematic coupling of the four planetary gearsets with each other and with the input and output shafts, a carrier of the fourth planetary gearset and the input shaft are connected, forming a first shaft of the gearbox; a carrier of the third planetary gearset and the output shaft are connected, forming a second shaft of the gearbox; a sun gear of the first planetary gearset and a sun gear of the fourth planetary gearset are connected, forming a third shaft of the gearbox; a ring gear of the first planetary gearset forms a fourth shaft of the gearbox; a ring gear of the second planetary gearset and a sun gear of the third planetary gearset are connected, forming a fifth shaft of the gearbox; a carrier of the first planetary gearset and a ring gear of the third planetary gearset are connected, forming a sixth shaft of the gearbox; a sun gear of the second planetary gearset and a ring gear of the fourth planetary gearset are connected, forming a seventh shaft of the gearbox; and a carrier of the second planetary gearset forms an eighth shaft of the gearbox. With respect to the kinematic coupling of the five shifting elements to the four planetary gearsets, and to the input and output shaft, the first shifting element is arranged in the power flow between the third shaft and a housing of the gearbox, the second shifting element between the fourth shaft and the housing of the gearbox, the third shifting element between the first and fifth shaft, the fourth shifting element either between the eighth and second shafts or between the eighth and sixth shafts, and the fifth shifting element either between the seventh and fifth shafts or between the seventh and eighth shafts, or between the fifth and eighth shafts.
As a general concept, automatically shiftable motor vehicle gearboxes in planetary design have been described many times before in the state of the art and are subject to constant further development and improvement. Gearboxes of this kind should therefore disclose a sufficient number of forward gears and one reverse gear, and a high transmission-ratio spread and favorable progressive ratio that are suited to motor vehicles. In addition, they should enable a high starting-torque ratio in a forward direction and include a direct gear for use in both passenger cars and commercial vehicles. In addition, these gearboxes should also be easy to install and should require, in particular, a small number of shifting elements, and they should avoid double shifting during sequential shifting, so that when shifting into defined shifting groups, only one shifting element is changed at any given time.
The present invention is based on the task of proposing a multi-speed gearbox of the initially described art, with at least eight forward gears that can be shifted without range-shifting and at least one reverse gear, which, with the use of at least four planetary gearsets, requires a minimum number of shifting elements. In addition, the gearbox should have a high transmission-ratio spread with relatively harmonic gear shifting and, at least in the main driving gears, should demonstrate a favorable degree of efficiency, i.e., relatively low towing and toothing losses.